Selective oxidation of propane to propylene oxide

ABSTRACT

The invention provides a one pot method for generating propylene oxide, the method having the steps of contacting propane with catalyst clusters no greater than 30 atoms in the presence of oxygen for a time sufficient to directly convert the propane to the propylene oxide. The invented method eliminates the generation of intermediate compounds or intermediate reaction steps.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Contract No.DE-AC02-06CH11357 awarded by the United States Department of Energy toUChicago Argonne, LLC, operator of Argonne National Laboratory. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a system and a method for producing propyleneoxide and more specifically, this invention relates to a system andmethod for producing propylene oxide directly from propane.

2. Background of the Invention

Propylene oxide (PO) is a key commodity of the petrochemical industry,produced from propylene, and is used to produce a wide range of consumerproducts like rigid foams, moldings, adhesive, coatings etc. PO has apresent market value of $13 billion and is projected to have a 50% riseby 2023.

A myriad of protocols for producing PO exist, just not directly frompropane. Protocols are available for transforming propylene to PO.However, those industrial techniques like chlorohydrin-, hydroperoxide-,cumene-based, and styrene monomer-processes produce abundant sideproducts and side streams.

A current route to produce PO, is by propylene epoxidation or by atwo-step conversion from propane. At the first step, propane isdehydrogenated to propylene and H₂. In the next step, the mixture ofpropylene, propane, and H₂, is co-fed with oxygen and then treated witha suitable catalyst (for e.g. titanium silicate) to form PO. Thesemulti-step processes are resource intensive and time consuming.

At present, there is no process or system for directly convertingpropane to PO.

A need exists in the art for a single step conversion process of propaneto propylene oxide. The process should bypass intermediate productgeneration, separation, and storage. The process and system should alsoeliminate the need for complex oxidation routes to produce PO by e.g.,hydrogen peroxide or cumene. Rather, the process and system shouldproduce PO at higher efficiencies using lower, cheaper metal loadings,compared to the state of the art processes, and at lower costs.

SUMMARY OF INVENTION

An object of the invention is to provide a system and method forproducing propylene oxide that overcomes many of the drawbacks of theprior art.

Another object of the invention is to provide a system and method forefficiently producing propylene oxide. A feature of the invention isutilization of catalysts comprised of specific numbers of atoms. Anadvantage of the invention is the high activity due to exposing propanefeedstock to every atom comprising the conversion catalysts utilized.For example, about 1.2 propylene oxide molecules are produced per metalatom per second at 300° C.

Still another object of the invention is to provide a one pot method forproducing PO from propane. A feature of the invention is that nearly 100percent selectivity to PO from about 150 C and 300° C. (For example, ifone propylene oxide molecule is produced for every propane molecule,that represents 100 percent selectivity) An advantage of the inventionis elimination of oxidative dehydrogenation of propane (which itself istypically performed at significantly higher temperatures), theelimination of precious metal catalysts or oxide based catalysts, andthe elimination of a separation sequence for propene.

Yet another object of the invention is to provide a low temperature, onestep process for producing PO. A feature of the invention is that ituses molecular oxygen in an exothermic one-step process. An advantage ofthe invention is the exothermicity of such oxidative dehydrogenation ismuch less energy demanding than endothermic non-oxidativedehydrogenation.

Another object of the invention is to provide a low cost process formaking propylene oxide. A feature of the invention is the use of highlydispersed metals, i.e. catalysts made of ultra-small particles(clusters) comprising a handful of atoms. These metals may consist ofabundant relatively inexpensive elements. An advantage of the inventionis that it makes the most efficient and economic use of precious metalsand common metals. For example, the invented method generates propyleneoxide directly from propane without any intermediate steps or reactionsand without first preparing propylene.

Briefly, the invention provides a one pot method for generatingpropylene oxide, the method comprising contacting propane with catalystclusters smaller than 1 nanometer in diameter (e.g., 1-30 atoms) in thepresence of oxygen for a time sufficient to convert the propane to thepropylene oxide.

BRIEF DESCRIPTION OF DRAWING

The invention together with the above and other objects and advantageswill be best understood from the following detailed description of thepreferred embodiment of the invention shown in the accompanyingdrawings, wherein:

FIGS. 1A-1B are a series of graphs showing rate of formation (TOR) andselectivity of reaction products using neat 4-atom based copper clusters(Cu₄), in accordance with features of the present invention;

FIGS. 2A-B are a series of graphs showing rate of formation (TOR) andselectivity of reaction products using neat 12-atom based copperclusters (Cu₁₂), in accordance with features of the present invention;

FIGS. 3A-3B are a series of graphs showing rate of formation (TOR) andselectivity of reaction products using neat 20-atom based copperclusters (Cu₂₀), in accordance with features of the present invention;

FIGS. 4A-B are a series of graphs showing rate of formation (TOR) andselectivity of reaction products using neat 4 atom palladium clustersPd₄, in accordance with features of the present invention;

FIGS. 5A-B are a series of graphs showing rate of formation andselectivity of reaction products using 4-atom copper, 1 atom palladiumclusters Cu₄Pd, in accordance with features of the present invention;

FIGS. 6A-B are a series of graphs showing rate of formation andselectivity of reaction products using 3-atom copper, 1-atom palladiumclusters Cu₃Pd, in accordance with features of the present invention;and

FIG. 7A-C are schematic depictions of the interactions of reagent andproduct molecules to atoms comprising the catalyst cluster, inaccordance with features of the present invention.

DETAILED DESCRIPTION

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings.

All numeric values are herein assumed to be modified by the term“about”, whether or not explicitly indicated. The term “about” generallyrefers to a range of numbers that one of skill in the art would considerequivalent to the recited value (e.g., having the same function orresult). In many instances, the terms “about” may include numbers thatare rounded to the nearest significant figure.

The recitation of numerical ranges by endpoints includes all numberswithin that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and5).

The following detailed description should be read with reference to thedrawings in which similar elements in different drawings are numberedthe same. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly stated. Asused in this specification and the appended claims, the term “or” isgenerally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising” or “having”an element or a plurality of elements having a particular property mayinclude additional such elements not having that property.

The present invention provides a method and system for the efficientconversion of propane to propylene oxide. A salient feature of theinvention is the use of clusters comprising mono- and/or bimetalliccatalysts to directly convert propane to propylene oxide in the presenceof oxygen. The clusters can be as large as 10 nm, but typically arebetween approximately 0.2 nm and 5 nm in diameter and preferably between0.3 nm and 7 nm in diameter.

These small clusters can serve as model catalytically active sites witha large fraction of undercoordinated, thus potentially highly active,sites.

The present invention reports a new catalyst to produce PO directly frompropane. The invention eliminates the multi-step process of need of 1)producing propylene from propane, 2) oxidizing the produced propylene toPO, and then 3) separating out any residual propylene from the reactionliquor. Since the reported process works with molecular oxygen feed, anefficient activation of oxygen by the reported catalysts occurs duringthe process.

The invented catalyst facilitates the following reactions:C₃H₈+1/2O₂→C₃H₆+H₂O  Equation 1C₃H₆+1/2O₂→C₃H₆O  Equation 2

A schematic of the above chemistry is depicted in FIGS. 7A-7C, which isdiscussed infra.

An embodiment comprises a subnanometer cluster based mono- andbi-metallic catalyst made of Cu and Pd that produces propylene oxidedirectly from propane with high activity and selectivity, in a one-pot(i.e., one reaction vessel) synthesis process. The invention leveragesthe temperature-dependent catalytic properties of the metal-oxidefilm-supported monometallic Cu and Pd clusters, as well as bimetallicCu—Pd clusters.

Metal oxide supports may include those selected from the groupconsisting of alumina, iron-oxide, silica oxide, zeolites, titaniumoxide, zinc oxide, zirconium oxide, tin oxide, magnesium oxide,including their combinations. Carbon based supports selected from thegroup consisting of nanocrystalline diamond, graphite, amorphous carbonvarious forms and compositions of graphene may also be utilized,including their combinations. These supports may be modified withoxygen, nitrogen, hydrogen and metal dopants. The supports may defineplanar or nonplanar surfaces or loose aggregate such as powders.

Ultimately, the supports are exposed to a gas mixture containing propaneand oxygen, at near atmospheric pressure.

The conversion occurs at high rates (up to about 1.2 propylene oxidemolecules produced per metal atom per second at 300° C.), withsuppressed formation of such combustion products as CO and CO₂. Catalystperformance is tunable through cluster-size and cluster-composition.

At higher temperatures the cluster-based process produces propylene,another high volume commodity chemical, at a high rate (up to about 1.6propylene molecules produced per cluster metal atom per second at 550°C.), with high selectivity (about 80 percent). From the studied Cu, Pdand CuPd cluster compositions, monometallic Cu clusters were foundpossessing the highest activity as well as selectivity in both propyleneoxide and propylene production. Theoretical calculations support theexperimentally observed high activity and selectivity of the bestperforming Cu₄ clusters. No quantifiable acrolein byproduct formationwas observed, as anticipated by theoretical calculations which showhigher barriers for acrolein production than for PO production.

Reaction Conditions Detail

Oxidative dehydrogenation of propane occurs over different size metalclusters. Reactant gas may comprise pure propane (i.e., neat withtypical impurities), or else in a carrier gas. Temperature of theconversion ranges from 150° C. to 300° C. Between temperature steps of50° C., a slow heating/cooling is applied to assure thermalstabilization. Suitable pressures are from 0.01 atm to 20 atm.

Feedstream components can vary, as long as propane comprises about 0.2percent or more of the feedstream, and oxygen comprises about 0.05percent or more of the feedstream. The presence of possible promoters,such as co-fed water, hydrogen, CO₂, N₂O, H₂O₂, O₃, and combinationsthereof can be used to further increase efficacy.

Catalyst Preparation

Detail

Small clusters (those containing less than about 30 atoms) comprisecatalytically active sites with a large fraction of undercoordinated,thus potentially highly active, sites. These features along with thestrong charge transfer with the support material and cluster'sfluxionality confers the clusters with features not present within itsbulk analog.

A myriad of elemental metals, their alloys and compounds may serve ascatalyst material, including but not limited to Cu, Ag, Au Co, Fe, Mo,Pd, Pt, Ti, V, W, their oxides and carbides, and combinations thereof.

The catalysts are prepared by softly landing the clusters which areproduced in a molecular beam within a high vacuum chamber on an ALDcoated substrate (e.g. alumina) on SiO₂/n-type (P-doped) Si wafer. Theclusters are so landed that the impact energy is less than 1 eV per atomwhich ensures that the clusters stay intact and does not undergofragmentation or pinning onto the substrate. Soft landing protocols aredescribed in U.S. Pat. No. 8,143,189 B2, issued to the applicant, andincorporated in its entirety herein.

Alternatively, cluster distributions may be prepared by wet methods,such as those methods described in Wentao Wei, Yizhong Lu, Wei Chen, andShaowei Chen J. Am. Chem. Soc., 2011, 133 (7), pp 2060-2063, theentirety of which is incorporated herein by reference.

Rigid substrates may be metal oxide selected from the group consistingof aluminum oxide, iron oxide, silicium oxide, zeolites, titanium oxide,zinc oxide, zirconium oxide, tin oxide, magnesium oxide, cerium oxideand combinations thereof. The substrate could be further doped withalkali metals. The clusters-catalysts themselves could be doped withalkali atoms as well.

Example 1

Two clusters within two spots of 8 mm diameter were deposited on the topof n-doped silicon wafers coated with a thin layer of alumina. Thealumina layer, of about 3 monolayer (ML) thickness, was fabricated byatomic layer deposition. The metal loading of the Cu₄, Cu₁₂, Cu₂₀, Pd₄,Cu₄Pd, and Cu₃Pd samples was 16.2 ng, 16.2 ng, 16.2 ng, 27.2 ng, 18.4ng, and 19.00 ng respectively, corresponding to a surface coverage of 10percent of an atomic monolayer equivalent. This ensures theinter-cluster distance of approximately 5-10 nm and inhibits anysintering occurring during the reaction as the catalyst is heated.

The reaction was performed in situ with X-ray characterization tosimultaneously monitor the reaction products formation on a massspectrometer, and to monitor the changes in the oxidation state of theclusters during the course of the reaction. The reactor was maintainedat a pressure of 800 Torr with a continuous 18.54 sccm flow of 3% O₂ and3% propane mixed in helium carrier gas.

Turnover rate (TOR) is defined as the number of product molecules formedper atom of the catalyst per second. TOR for PO production reached 1.2at 300° C. which matches or is significantly higher than that obtainedby the to-date used techniques where, however, propylene or a mixture ofpropane and propylene are used as the starting gases.

FIG. 1A depicts the TOR for propylene, CO and CO₂ for neat 4-atom copperclusters (Cu₄). FIG. 1B depicts the carbon based selectivity for thereaction products for 4-atom clusters.

FIG. 2A depicts the TOR for propylene, CO and CO₂ for neat 12-atomcopper clusters (Cu₁₂). FIG. 2B depicts the carbon based selectivity forthe reaction products for 12-atom clusters.

FIG. 3A depicts the TOR for propylene, CO and CO₂ for neat 20-atomcopper clusters (Cu₂₀). FIG. 3B depicts the carbon based selectivity forthe reaction products for 20-atom clusters.

The TORs obtained for the larger Cu clusters featured in FIGS. 2A-B and3A-3B (Cu₁₂, and Cu₂₀ atoms) are comparable to those observed for Cu₄clusters. This relaxes the specific size requirement for high activityof the catalysts, therefore providing a means for large scale productionusing alternative techniques, including catalysts prepared by“wet-chemistry.”

This data shows 100 percent selectivity for propylene oxide at 150° C.and 300° C., which is to say no combustion products or unwanted moietiesare generated.

Example 2

FIGS. 4A, 4B, 5A, 5B, 6A, and 6B depict the formation rate of propyleneoxide per metal atom and selectivity on neat 4-atom Pd catalyst clusters(FIGS. 4A-4B) and Cu—Pd catalyst clusters (FIGS. 5A, 5B, 6A, and 6B).

TOR for propylene, propylene oxide, CO and CO₂ are plotted for 4-atompalladium clusters Pd₄ (FIG. 4A), 4-atom copper, 1 atom palladiumcluster Cu₄Pd (FIG. 5A), and 3-atom copper, 1-atom palladium clusterCu₃Pd (FIG. 6A). Carbon based selectivity for the reaction products areplotted for the Pd₄ (FIG. 4B), Cu₄Pd (FIG. 5B), and the Cu₃Pd (FIG. 6B).

FIGS. 7A-7C is a schematic depiction of the interaction of reagent andproduct molecules to atoms comprising the catalyst cluster. FIG. 7Ashows the propane (in a box) adsorbed onto the catalyst. FIG. 7B showspropylene intermediate adsorbed onto the catalyst. This is a transientcondition, with the propylene (showed in a dashed box) converting as itis formed onto the catalyst. FIG. 7C shows propylene oxide product (in abox) formed on the catalyst. The arrows in the three panels designatethe atoms in the cluster to which the substrate propane (FIG. 7A) andthe product propylene oxide (FIG. 7C) dock.

Upon formation, the product propylene oxide desorbs from the catalystsurface and remains in gas phase for harvesting via typical negativepressure or other means.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting, but are instead exemplaryembodiments. Many other embodiments will be apparent to those of skillin the art upon reviewing the above description. The scope of theinvention should, therefore, be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the terms“comprising” and “wherein.” Moreover, in the following claims, the terms“first,” “second,” and “third,” are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. § 112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function void of further structure.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” “more than”and the like include the number recited and refer to ranges which can besubsequently broken down into subranges as discussed above. In the samemanner, all ratios disclosed herein also include all subratios fallingwithin the broader ratio.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, thepresent invention encompasses not only the entire group listed as awhole, but each member of the group individually and all possiblesubgroups of the main group. Accordingly, for all purposes, the presentinvention encompasses not only the main group, but also the main groupabsent one or more of the group members. The present invention alsoenvisages the explicit exclusion of one or more of any of the groupmembers in the claimed invention.

The embodiment of the invention in which an exclusive property orprivilege is claimed is defined as follows:
 1. A one pot method forgenerating propylene oxide, the method comprising: a) depositing aplurality of catalyst clusters onto a substrate, wherein the depositedclusters comprise no more than 30 atoms of metal and contain no oxide;b) contacting propane with the catalyst clusters in the presence ofoxygen to convert the propane to the propylene oxide.
 2. The method asrecited in claim 1 wherein the metal is selected from the groupconsisting of copper, palladium, platinum, silver, gold, cobalt, andcombinations thereof.
 3. The method as recited in claim 1 wherein thecatalysts are rigidly positioned, relative to each other.
 4. The methodas recited in claim 1 wherein the substrate is a rigid substrate.
 5. Themethod as recited in claim 4 wherein the rigid substrate is a metaloxide selected from the group consisting of aluminum oxide, iron-oxide,silica oxide, zeolites, titanium oxide, zinc oxide, zirconium oxide, tinoxide, magnesium oxide, cerium oxide and combinations thereof.
 6. Themethod as recited in claim 1 wherein substrate is loose aggregate. 7.The method as recited in claim 1 wherein the method is conducted in aclosed reaction vessel and the propane and oxygen are entrained in acarrier gas flowing through the vessel.
 8. The method as recited inclaim 7 wherein the carrier gas is a relatively inert gas selected fromthe group consisting of nitrogen, argon, helium, and combinationsthereof.
 9. The method as recited in claim 1 wherein the method isconducted at ambient pressure.
 10. The method as recited in claim 1wherein the method is conducted at pressures ranging from between about0.01 atm and 20 atm.
 11. The method as recited in claim 1 wherein themethod is conducted at temperatures between about 25° C. and 400° C. 12.The method as recited in claim 1 wherein the selectivity for propyleneoxide is at least 50 percent at reaction temperatures of between about25° C. and 400° C.
 13. The method as recited in claim 1 wherein thecatalyst comprises copper and the clusters contain between 1 and 30atoms.
 14. The method as recited in claim 6 wherein the aggregate isfluidized.
 15. The method as recited in claim 1 wherein propylene oxideis generated from propane without any intermediate reaction steps. 16.The method as recited in claim 1 wherein propylene oxide is generatedfrom propane without the production of intermediates.
 17. The method asrecited in claim 4 wherein the rigid substrate is a carbon based supportselected from the group consisting of nanocrystalline diamond, graphite,amorphous carbon, graphene, and combinations thereof.
 18. The method asrecited in claim 1 wherein the clusters further compriseundercoordinated active sites.
 19. The method as recited in claim 1wherein the clusters are deposited as neat metal.